| Summary: | The high variability of aerosol particle concentra- tions, sizes and chemical composition makes their descrip- tion challenging in atmospheric models. Aerosol–cloud in- teraction studies are usually focused on the activation of accumulation mode particles as cloud condensation nuclei (CCN). However, under specific conditions Aitken mode par- ticles can also contribute to the number concentration of cloud droplets (Nd), leading to large uncertainties in pre- dicted cloud properties on a global scale. We perform sen- sitivity studies with an adiabatic cloud parcel model to con- strain conditions under which Aitken mode particles con- tribute to Nd. The simulations cover wide ranges of aerosol properties, such as total particle number concentration, hy- groscopicity (κ) and mode diameters for accumulation and Aitken mode particles. Building upon the previously sug- gested concept of updraft (w)- and aerosol-limited regimes of cloud droplet formation, we show that activation of Aitken mode particles does not occur in w-limited regimes of ac- cumulation mode particles. The transitional range between the regimes is broadened when Aitken mode particles con- tribute to Nd, as aerosol limitation requires much higher w than for aerosol size distributions with accumulation mode particles only. In the transitional regime, Nd is similarly de- pendent on w and κ. Therefore, we analyze the sensitivity of Nd to κ, ξ(κ), as a function of w to identify the value combinations above which Aitken mode particles can af- fect Nd. As ξ(κ) shows a minimum when the smallest ac- tivated particle size is in the range of the “Hoppel mini- mum” (0.06 µm ≤ Dmin ≤ 0.08 µm), the corresponding (w– κ) pairs can be considered a threshold level above which Aitken mode particles have significant impact on Nd. This threshold is largely determined by the number concentration of accumulation mode particles and by the Aitken mode di- ameter. Our analysis of these thresholds results in a simple parametric framework and criterion to identify aerosol and updraft conditions under which Aitken mode particles are ex- pected to affect aerosol–cloud interactions. Our results con- firm that Aitken mode particles likely do not contribute to Nd in polluted air masses (urban, biomass burning) at mod- erate updraft velocities (w ≤ 3 m s −1 ) but may be important in deep convective clouds. Under clean conditions, such as in the Amazon, the Arctic and remote ocean regions, hygro- scopic Aitken mode particles can act as CCN at updrafts of w < 1 m s −1 .
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